Catalysts positioned in the exhaust of engines act to convert emissions produced during combustion to compounds which may be released to the atmosphere. Frequently, these catalysts convert emissions most efficiently at elevated exhaust temperatures, and thus during engine cold start and warm-up conditions when the catalyst is below normal operating temperature, emissions may slip past the catalyst. One method to rapidly heat the catalyst includes increasing the temperature of exhaust gas traveling through the catalyst.
One example approach for increasing exhaust gas temperature is described in U.S. Patent Application Publication 2010/0005784. In the cited reference, an exhaust back-pressure valve positioned in the exhaust of an engine is closed to increase the exhaust temperature in order to perform a desulfination of a catalyst.
However, the inventors herein have recognized a few issues with the above approach. Increasing exhaust back-pressure by closing the exhaust back-pressure valve may increase the dilution of the cylinder charge with burned gases, resulting in unstable combustion under some conditions. Further, the diluted cylinder charge may limit the amount of spark retard that can be applied, thus limiting the temperature increase of the exhaust.
Accordingly, one embodiment to at least partly address the above issues includes a method for operating an engine. The method comprises closing an exhaust back-pressure valve in response to a component temperature, and adjusting intake and/or exhaust valve operation in response to closing the exhaust back-pressure valve to reduce cylinder internal exhaust gas recirculation.
In this way, the exhaust gas temperature may be increased responsive to a component temperature (e.g., a temperature of a catalyst being below a threshold) by throttling the exhaust via the exhaust back-pressure valve. Additionally, intake and/or exhaust valve timing may be adjusted to reduce the trapping of exhaust residuals in the cylinders, thus reducing the internal exhaust gas recirculation (e.g., dilution of the cylinder charge) and increasing combustion stability. In some examples, external exhaust gas recirculation may also be decreased to further reduce the dilution of the cylinder charge. The closure of the exhaust back-pressure valve may also be restricted to high-combustion stability operating conditions, such as low humidity and low altitude, to avoid increased combustion instability.
The present disclosure may offer several advantages. For example, rapid catalyst heating may be attained without compromising combustion stability, thus avoiding degraded exhaust emissions and potential misfires. By rapidly heating the catalyst, exhaust emissions during engine cold starts may be reduced. Additionally, by maintaining relatively low dilution even with the exhaust back-pressure valve closed, spark timing may be further retarded to increase the exhaust gas temperature.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.